class factory

Alex Martelli aleax at
Wed Aug 20 23:37:05 CEST 2003

Nick Keighley wrote:

> Hi,
> I'm a Python beginner and I'm would like to write a function that
> returns a class (perhaps a tad ambitious...). I've looked through
> the FAQ and perused "Python In  A Nutshell" (not a good book to
> start with?). The only example I found in PiaN used a simple

If you're an experienced programmer with other languages then
the Nutshell should be helpful -- if you're a newbie to programming,
you should start with easier books (but with the kind of tasks
you're setting yourself I guess you aren't).  I didn't particularly
emphasize metaprogramming in the Nutshell -- it's more of a "gee
whiz" kind of thing and the Nutshell aims to cover solid, everyday,
bread-and-butter usage.

> if statement to return one of a selection of pre-existing classes.
> I'd like to generate a class on-the-fly from a parameter (a dictionary).

How do you want to use that dict -- as the class's dict?  Then either:

def makeaclass(fromdict):
    class X: pass
    X.__dict__ = fromdict
    return X


def makeaclass(fromdict):
    class X: pass
    return X

might be helpful.

> Can Python do this sort of stuff? Does the mean I have to mess with
> the dreaded meta-classes?

Metaclasses may be easier to use than sufficiently-general class-building
functions, actually -- no reason to dread them.  But anyway, yes, Python
IS pretty good at metaprogramming, too, both with and without metaclasses.


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